Irradiation of a target volume in an irradiation volume with ion or particle beams concerns the irradiation of matter, in particular inorganic, organic and biological materials, and is used in various fields of research, industry and medical engineering. The target volume includes, in particular, the region in which a predetermined dose is to be deposited in order to modify the irradiated material; the irradiation volume also includes, in particular, those regions of the material which are penetrated by radiation in order to achieve the desired dose in the target volume. A particle beam or ion beam is understood, in particular, as a high-energy beam of either charged particles, e.g. protons, carbon ions or ions of other elements, pions or neutral particles, e.g. neutrons. In the following description, the terms ion beam and particle beam are used interchangeably. High energy is understood, in particular, as energy of the particles in the region of several MeV/amu up to several GeV/amu (amu: atomic mass unit).
An irradiation device which is suitable for carrying out the irradiation in general has an acceleration facility which generates and forms the ion beam, the ion beam being guided for the irradiation via a beam transport system into a region in which the irradiation volume is arranged. The irradiation device also includes a beam modification facility, which can adapt the parameters of the ion beam to the position and size of the target volume.
The irradiation volume can, for example, be a detector system, which is used to verify an irradiation field. In general, the irradiation volume includes an irradiation field, which is a field with maximum extent in the lateral direction, in general in the x and y directions, and is perpendicular to the direction of the ion beam. The detector system can consist of a detector field or a so-called stack, with multiple laterally extended detector fields arranged one behind the other. In the dosimetry field, for example, films with a photographic emulsion are used for this purpose. Nuclear trace detectors are also used to measure the fluence distribution in the irradiation field. In the field of medical applications, irradiation of biological tissue is used to study the action of particle radiation, in order to be able to estimate the action of exposure to beams of cosmic radiation in space. Finally, the irradiation volume can also be the volume of a tumour in a patient. In this case, ion beams are used to destroy tumour tissue in the target volume.
In tumour therapy, the special properties of ion beams make it possible to destroy the tumour tissue with minimal damage to the surrounding healthy tissue. This is associated with the favourable depth dose distribution of ion beams. When high-energy ion beams penetrate the material, at first they deposit little energy. With increasing depth, the energy deposition increases, reaches its maximum in the region of a distribution curve called the Bragg peak, and then falls steeply. In this way, even in the case of deeper tumours, more energy can be deposited in the tumour tissue than in the surrounding healthy tissue.
Ion beams have an action on the irradiation volume depending on the type of material to be irradiated and the parameters of the ion beam. In general, ion beams have a different action from photon radiation. This means that with ion beams the dose to be deposited is different from with photon beams, in order to achieve a predetermined action or predetermined irradiation effect. The photon dose Dγ which would cause the same irradiation effect as the ion dose DI is designated as the effective dose. The changed action of ion beams is observed for inorganic, organic and biological material. In inorganic materials, a smaller action of ion beams compared with photon beams tends to be observed. In contrast, when biological material is irradiated with ions, usually a higher action and thus a greater effect compared with photon irradiation is observed.
Before the actual irradiation, in general an irradiation plan for irradiating the target volume, e.g. a sub-region in a phantom or tumour, is produced. In the case of irradiation with ion beams, this irradiation plan should take into account as far as possible the action of ion beams.
Various methods for producing an irradiation plan are known. For example, in the publication Krämer and Scholz 2000, Physics in Medicine and Biology, Vol. 45, pp. 3319-3330, a method for producing an irradiation plan is described.
The action of ion beams in the material depends in a complex manner on the ion type, the ion energy, the irradiation dose, the irradiated material and the observed effect in each case. Experimental determination of these multiple dependencies with the necessary precision for irradiation planning is unachievable in practice. Models which allow prediction of the changed effectiveness therefore represent an important tool for implementation of irradiation planning. These models are usually based on simplifications and approximations, since the mechanisms on which they are based for damaging inorganic, organic and biological material are not yet clarified quantitatively with sufficient precision. Correspondingly, in general the application field of the models is also limited.
An example of such a model is described in the publication Scholz et al., Radiation Environmental Biophysics, Vol. 36, pp. 59-66 (1997). The model is called LEM, which is an abbreviation of “local effect model”.
The models until now cannot supply any sufficiently precise information for irradiation planning over the whole range from light to heavy ions.